This invention relates to the provision of spectrally selective reflectors in optical waveguides, particularly reflectors of the Bragg grating type. It is known to create such gratings in optical fibre by lateral irradiation of such a fibre with a fringe pattern of relatively high intensity ultra-violet light, relying upon the photorefractive effect to cause that light to induce a corresponding pattern of localised refractive index change. The construction of a spectrally selective reflector to conform reasonably closely to a desired spectral profile requires correspondingly close control over absolute local refractive index values, apodisation and uniformity across the waveguide. This is difficult to achieve in using the photorefractive effect to write a Bragg grating in an optical fibre by the lateral illumination method referred to above. This is partly because the grating has to be written in the optical core through the cladding, and partly because the photorefractive effect is not easy to harness in the material system being used.
For the manufacture of spectrally selective reflectors in optical waveguides that are not optical fibre waveguides, a number of techniques have been proposed which involve applying a layer of mask material to the core of the waveguide, patterning this material to provide a set of windows for modifying the underlying core by selective ion exchange diffusion of a refractive index modifying dopant through the windows, or by selective etching through the windows to form trenches which are then infilled with lower refractive index material.
The etch and infill approach is for instance described in U.S. Pat. No. 5 195 161, which briefly alludes to photolithographic masking preparatory for etching a series of recessed features (trenches) by reactive ion etching, and then infilling them with material of lower refractive index. A similar infilling approach appears to be described also in the abstract of JP 63-106605 A appearing in Patent Abstracts of Japan, vol. 12, no. 354 (P-761), though the abstract does not specifically identify the grooves as being formed by ion etching. The ion exchange diffusion approach is for instance described in U.S. Pat. No. 5 080 503, which describes depositing a film of masking material upon a substrate in which a waveguide has been formed, opening windows in the mask material and then immersing the masked substrate in a bath of molten salt to induce ion exchange through the windows in the mask. Additionally, though not specifically in the context of Bragg reflective gratings in optical waveguides, but instead in the context of Bragg diffractive gratings in optical waveguides, U.S. Pat. No. 4 262 996 briefly states that, in the preferred embodiment, the index modulation providing the grating structure is accomplished on the surface of the optical waveguide by corrugation or etching (chemical, plasma, ion beam, etc.), by overlay, or by diffusion of dopants into the material of the optical waveguide.
For the creation of optical waveguide Bragg reflection gratings, the etch and infill approach, also the ion exchange diffusion approach, and diffusion, are all unattractive compared with the photorefractive effect approach in relation to the definition and control attainable having regard to the fact that the pitch of such a grating is liable to be only about 500 nm.